This invention relates to the alloy art and has particular relationship to superalloys and the method of heat treating these alloys. Superalloys are alloys having high strength at elevated temperatures.
This invention arises out of experience with nuclear reactors particularly of the fast breeder, liquid metal type. Notwithstanding the origin of this invention, it is to be understood that this invention is in no respect confined in its scope to nuclear reactors. To the extent that the alloys created pursuant to this invention or the method by which these alloys are processed may find general use; such use is within the scope of the invention.
In fast breeder nuclear reactors of the liquid metal type, as well as in others, the fuel is encapsulated in cladding, typically of cylindrical form. A capsule containing the fuel is usually referred to as a fuel element or fuel rod. In accordance with the teachings of the prior art, the cladding is composed of stainless steel, typically AISI 316 stainless steel. The ducts through which the liquid metal (typically sodium) flows are also composed of this 316 steel. In practice, difficulty has been experienced both with the cladding and the ducts. The stainless steel on being bombarded by neutrons, particularly where the neutron flux is epithermal (E&gt;0.1 MeV), swells. In addition, the stainless steel does not have the necessary strength at the elevated temperatures, 500.degree. C. and higher, at which the reactors of the type involved operate. The problem is particularly serious in the case of the cladding. On being heated by the fission reaction, the fuel in the capsules expands and in addition gas is generated and exerts high pressure at the high temperatures within the capsules. The cladding is highly stressed. The stress exerted in the ducts is at a lower level both because the temperature of the ducts is lower than that of the cladding and also because the mechanical pressure to which the ducts are subjected is lower. The stainless steel of the cladding and of the ducts is subject to substantial creep which is accentuated by the neutron irradiation.
Various alloys have been considered in efforts to provide improved cladding and duct materials. Among the alloys studied have been gamma prime hardened austenitic superalloys such as those described in U.S. Pat. Nos. 3,199,978; 4,129,462; and 4,172,742. In addition to the metallurgical conditions described in the listed patents, these alloys have also been studied in a solution treated and cold worked condition. These gamma prime austenitics can generally be designed to have good swelling resistance, high strength and high stress rupture strength relative to austenitic alloy 316.
Another class of alloys under consideration for use as cladding and duct material are the fully ferritic precipitation hardening alloys containing little, if any, nickel. Examples of these alloys are described in U.S. Pat. No. 4,049,431. It is believed these alloys, when properly treated, can provide a combination of swelling resistance, acceptable ductility and high strength at the temperature typically encountered by liquid metal fast breeder reactor cladding.